Autoradiography has been used to evaluate lymphocyte proliferation in the neonatally thymectomized rat in comparison with the normal animal. The data obained show that the proliferative activity of lymphocytes is greatly increased in the thymus-dependent areas 4–6 weeks after thymectomy, whereas it is normal or slightly increased 3 months laters. It seems plausible to assume that a thymus factor or chalones in situ produced by specific cells normally regulate the proliferation of thymus-derived cells. The increase of the proliferative activity accounts for the repopulation of the thymus-dependent areas, which are completely replenished in the older animals. Recirculation of the thymus cells is not confined to the thymus-dependent areas.